1. Field of the Invention
This invention relates to a system for driving a display device including photo elements in which the voltage for firing the photo element (which is hereinafter termed a firing voltage V.sub.B) or the operating voltage (which is hereinafter termed V.sub.A) differs from the voltage for erasing the luminescene (which is hereinafter termed an extinction voltage V.sub.M). That is, the invention is directed to a display device of the type wherein the relation of voltages is V.sub.M &lt; V.sub.B or V.sub.M &lt; V.sub.A.
An element which operates in accordance with the relation V.sub.M &lt; V.sub.B is the known gas discharge photo element, a photo diode having a negative resistance characteristic. An electro-luminescence element having a positive resistance characteristic, as well known, provides the relation V.sub.M &lt; V.sub.A.
Although the following explanation is directed to the gas discharge photo element (which is hereinafter called a discharge element for simplicity), such explanation is also applicable to the other above-described elements.
In conventional driving systems for the discharge elements, stray capacitance formed between the two electrodes of the discharge element is charged by pulsed current from a pulsed current source and the charge of the discharge element is discharged when the charge voltage of the stray capacitance rises above the firing voltage V.sub.B. The discharge of the discharge element is erased when the voltage of the stray capacitance falls below the extinction voltage V.sub.M after cut off of the pulsed current.
In such a system, a ratio of maximum luminance of the discharge element to minimum luminance thereof is almost 3:1 because of very high minimum luminance. Therefore, such a system has a defect that the contrast of displayed picture is insufficient. This defect becomes more apparent as the value of the stray capacitance, etc. become larger.